There are many known types of welding power supplies. One known type of welding power source is a phase controlled power source and includes a power circuit having controllable switches, such as SCRS, that are phase controlled. The phase controlled power supplies are relatively simple, robust and inexpensive.
Generally, phase controlled power supplies receive an ac input and provide an ac output, a rectified ac output, or a dc output. They control the amount of power provided by controlling the phase of an input ac cycle at which the controllable switches are turned on. Typically, the switches turn off at the next zero crossing. Thus, the portion of the input cycle from the on time to the zero crossing is provided as an output. (AC cycle length, as used herein, is the length of time from one zero crossing until the next zero crossing, or the length of time from one SCR being turned on, until the next SCR is turned on). A greater voltage output is provided by turning on earlier in each cycle, thus providing a greater portion of the input cycle as the output. A lesser output voltage is provided by turning on later in the cycle, and providing a lesser portion of the input cycle as the output. Current control can be obtained by feeding back output current to the phase angle regulator.
One such known phase control power supply is the Miller Mark VI.TM. power source. The Miller Mark VI.TM. power source is a multi-user power source that includes six phase controlled power circuits, one for each user output. The power circuits provide a dc cv (constant voltage) or cc (constant current)output. The power circuits include an SCR phase controlled rectifying bridge that receives an ac input. The output of the bridge is provided to an output circuit that includes an inductor to smooth the output. A controller receives a feedback signal, and compares the feedback signal to a user setpoint. The phase of the SCR firing is controlled in response to the comparison.
When the welding arc is initially created, a relatively high voltage (80V e.g.) is desirable. The relatively high voltage helps stabilize and create the arc. However, a higher voltage will possibly cause the user discomfort in the event the user touches both outputs. Accordingly, it is well-known in the art to provide circuitry which helps create a reduced open circuit voltage. Thus, when welding is not occurring the output voltage is reduced to a level which is less likely to cause the user discomfort. When the welding process begins a higher voltage is provided which is sufficient to start the arc. One such prior art circuit is shown in U.S. Pat. No. 5,513,093 (Corrigall et al), which is assigned to the owner of this invention, and is hereby incorporated by reference.
Other attempts have been made in prior art to provide a reduced open circuit voltage. However, such prior art attempts are generally not well-suited to use with a phase controlled power supply, are relatively complicated or do not provide a sufficiently reduced voltage without interfering with arc starting.
Accordingly, a phase controlled welding power supply that provides a reduced open circuit output voltage is desirable. Additionally, the reduced open circuit voltage should not interfere with arc starting and arc stability.